Thrusters of said type are known from the prior art. Such systems have become a very important piece of equipment, allowing movements of floating vehicles to be facilitated, above all, but not exclusively, in the marine field.
By the installation of one or more of these tunnel thrusters in the quickwork of a vessel, a boat, a watercraft, or a floating transport or working vehicle, it becomes possible not only to increase maneuvering and evolution ability of the vehicle upon which they are mounted, but it is also possible to help in the implementation of their dynamic positioning system.
Such systems are generally arranged transverse to the fore-and-aft axis of the marine unit in the quickwork of the hull and the tubular duct comes out at the sides of the hull where apertures are provided coinciding with the ends of said tunnel. Still generally, the axis of rotation of the propeller inside the duct is arranged transverse to the fore-and-aft axis of the marine vehicle.
Therefore, by the rotation of the propeller, a hydrodynamic force is generated which allows the marine unit to be turned or moved sideways as it is necessary during maneuvers such as for instance docking.
Because of the limited draft of the hull often it is not possible to install the propeller having the most suitable diameter in relation to the required power and to the overall length of the tunnel. In these cases the thrust values that can be obtained will be always lower than the maximum values that can be obtained in relation to the installed power.
In order to overcome such drawback, one tries to avoid hydrodynamic turbulences on the inner walls of the duct, since the thrust supplied by the propeller decreases more and more as the length relative to the diameter of the duct increases, due to major head losses throughout the walls of the tunnel and due to the local ones at the ends met by the water flow generated by propeller action.
A known solution is described in several documents, such as document GB112094, document U.S. Pat. No. 3,400,682 and document EP0037865. In these documents different arrangements of marine tunnel thrusters comprising propellers housed into ducts are described. These ducts provide two end flares, arranged at the ends of the duct respectively.
Such arrangement would allow hydrodynamic conditions on the inner walls of the duct near the end sides thereof to be improved. Notwithstanding this, such arrangement does not eliminate fractional resistances of the flow along the inner walls of the duct, which are still able to reduce the propeller thrust, since in order to reach the thrust required by the vessel for the manoeuvre a greater amount of power must be used.